An engine exhaust noise silencer for use in an automobile, wherein a valve which responds to the exhaust pressure is provided to change the exhaust flowpath, is disclosed for example in Tokkai Hei 5-202729.
In this device, the interior of a muffler is divided into a second expansion chamber, first expansion chamber and volume chamber in order of proximity to an exhaust manifold of the engine.
One end of an inlet tube opens out into the volume chamber, and connects with the first expansion chamber via a porous portion provided in a middle section. The first expansion chamber is connected with the second expansion chamber via a first internal tube, and the second expansion chamber is connected with the atmosphere via a tail tube. In addition, the volume chamber and second expansion chamber are connected via a second internal tube which passes through the first expansion chamber, and a valve which responds to the pressure in the volume chamber is provided at the outlet of the second internal tube.
When the valve is closed, exhaust which has flowed into the first expansion chamber from the porous portion of the inlet tube passes through the internal tubes and second expansion chamber to be discharged from the tail tube into the atmosphere. This flowpath is referred to as a first exhaust flowpath.
The downstream part of the inlet tube, the volume chamber and the second internal tube, in which exhaust does not pass when the valve is closed, form a resonance system which dampens the noise of this first exhaust flowpath.
On the other hand when the exhaust pressure in the volume chamber increases due to increase of the exhaust flowrate, the valve is open, and a part of the exhaust in the inlet tube takes a second exhaust flowpath comprising the second expansion chamber, the volume chamber and internal tube. As a result of this second exhaust flowpath, exhaust pressure losses which occur when the exhaust flowrate increases, for example at high engine speeds or under high load, are reduced. If the noises of the two exhaust flows which meet in the second expansion chamber, are arranged to have opposite phase, noise reduction can be obtained due to mutual interference in the confluence zone. This may be achieved by appropriately setting the resonant frequency of the resonance system. However in this noise silencer, as the resonance system has only one degree of freedom with respect to the first exhaust flowpath, satisfactory silencing performance cannot be obtained.
Another method to improve the silencing performance of the silencer is to decrease the diameter of the internal tubes, but in this case exhaust pressure losses increase.
Further, this silencer comprises two expansion chambers, but the volume of each chamber is lower so that silencing of low frequency noise is not very efficient.